The "Number of audit units to audit" is based on the NEGEXP method, which is very efficient and requires selecting larger audit units
with higher probability than smaller ones.
The numbers given here are based on a confidence
level of 75%. I.e. they are designed so the
audit will either 1) find a discrepancy and call for an escalation or
full hand recount, or 2) reduce the risk of confirming an incorrect
outcome to (100 - 75)%, even if the tally system has been manipulated.
A maximum "within-precinct-miscount" of 20% is assumed.
See
On Auditing Elections When Precincts Have Different Sizes, by Javed
A. Aslam, Raluca A. Popa and Ronald L. Rivest